1. Field of the Invention
This invention relates to the pressure vessel of a pressurized water reactor system of an advanced design in which plural rod guides are cantilever-mounted at their lower ends and extend in parallel, vertical relationship to dispose the upper ends thereof adjacent a calandria assembly and, more particularly, to a simplified, flexible top end support for the cantilever-mounted rod guides of such a pressurized water reactor.
2. State of the Relevant Art
Conventional pressurized water reactors employ a number of control rods which are mounted within the reactor vessel, generally in parallel axial relationship, for axial translational movement in telescoping relationship with the fuel rod assemblies. The control rods contain materials which absorb neutrons and thereby lower the neutron flux level within the core. Adjusting the positions of the control rods relatively to the respectively associated fuel rod assemblies thereby controls and regulates the reactivity and correspondingly the power output level of the reactor. Typically, the control rods, or rodlets, are arranged in clusters, and the rods of each cluster are mounted at their upper ends to a common, respectively associated spider. Each spider, in turn, is connected to a respectively associated adjustment mechanism for raising or lowering the associated rod cluster.
In certain advanced designs of such pressurized water reactors, there are employed both control rod clusters (RCC) and water displacer rod clusters (WDRC), and also so-called gray rod clusters which, to the extent here relevant, are structurally identical to the RCC's and therefore both are referred to collectively hereinafter as RCC's. In an exemplary such reactor design, a total of over 2800 reactor control rods and water displacer rods are arranged in 185 clusters, each of the rod clusters having a respectively corresponding spider to which the rods of the cluster are individually mounted. Further, there are provided, at successively higher, axially aligned elevations within the reactor vessel, a lower barrel assembly, an inner barrel assembly and a calandria assembly, each of generally cylindrical configuration; a removable, upper closure dome seals the top of the vessel and is removable to gain access to the vessel interior.
The lower barrel assembly has mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies, comprising the reactor core. The fuel rod assemblies are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates. The inner barrel assembly comprises a cylindrical sidewall which is welded at its bottom edge to the upper core plate. Within the inner barrel assembly there are mounted a large number of rod guides disposed in closely spaced relationship, in an array extending substantially throughout the cross-sectional area of the inner barrel assembly. The rod guides are of first and second types, respectively housing therewithin the reactor-control rod clusters (RCC) and the water displacer rod clusters (WDRC); these clusters, as received in telescoping relationship within their respectively associated guides, generally are aligned with respectively associated fuel rod assemblies.
One of the main objectives of the advanced design, pressurized water reactors to which the present invention is directed, is to achieve a significant improvement in the fuel utilization efficiency, resulting in lower overall fuel costs. Consistent with this objective, the water displacement rodlet clusters (WDRC's) function as a mechanical moderator and provide spectral shift control of the reactor. Typically, a fuel cycle is of approximately 18 months, following which the fuel must be replaced. When initiating a new fuel cycle, all of the WDRC's are fully inserted into association with the fuel rod assemblies, and thus into the reactor core. As the excess reactivity level of the fuel diminishes over the cycle, the WDRC's are progressively, in groups, withdrawn from the core so as to enable the reactor to maintain the same reactivity level, even though the reactivity level of the fuel rod assemblies is reducing due to dissipation over time. Conversely, the control rod clusters are moved, again in axial translation and thus telescoping relationship relatively to the respectively associated fuel rod assemblies, for control of the reactivity and correspondingly the power output level of the reactor on a continuing basis, for example in response to load demands, in a manner analogous to conventional reattor control operations.
A reactor incorporating WDRC's is disclosed in U.S. patent application Ser. No. 217,503, filed Dec. 16, 1980 and entitled MECHANICAL-SPECTRAL SHIFT REACTOR and in further applications cited therein. A system and method for achieving the adjustment of both the RCC s and WDRC's are disclosed in the co-pending application of Altman et al., entitled "DISPLACER ROD DRIVE MECHANISM VENT SYSTEM." Each of the foregoing applications is assigned to the common assignee hereof and is incorporated herein by reference.
A critical design criterion of such advanced design reactors is to minimize vibration of the reactor internals structures, as may be induced by the core outlet flow as it passes therethrough. A significant factor for achieving that criterion is to maintain the core outlet flow in an axial direction throughout the inner barrel assembly of the pressure vessel and thus in parallel axial relationship relative to the rod clusters and associated rod guides. The significance of maintaining the axial flow condition is to minimize the exposure of the rod clusters to cross-flow, a particularly important objective due both to the large number of rods and also to the type of material required for the WDRC's, which creates a significant wear potential. This is accomplished by increasing the vertical length, or height, of the vessel sufficiently such that the rods, even in the fully withdrawn (i.e., raised) positions within their inner barrel assembly, remain located below the vessel outlet nozzles, whereby the rods are subjected only to axial flow, and through the provision of a calandria assembly, which is disposed above the inner barrel assembly and thus above the level of the rods and which is disposed to withstand the cross-flow conditions.
In general, the calandria assembly comprises a lower calandria plate and an upper calandria plate which are joined by a cylindrical side wall, and an annularly flanged cylinder which is joined at its loeer cylindrical end to the upper calandria plate and is mounted by its upper, annularly flanged end on an annular supporting ledge of the pressure vessel. The rod guides are cantilever-mounted at their lower ends to the upper core plate and at their upper ends to the lower calandria plate. Within the calandria assembly and extending between aligned apertures in the lower and upper calandria plates is mounted a plurality of calandria tubes, positioned in parallel axial relationship and respectively aligned with the rod guides. A number of flow holes are provided in the lower calandria plates, at positions displaced from the apertures associated with the calandria tubes through which the reactor core outlet flow passes as it exits from its upward passage through the inner barrel assembly. The calandria assembly receives the axial core outlet flow, and turns the flow from the axial direction through 90.degree. to a radially outward direction for passage through the radially oriented outlet nozzles of the vessel. The calandria thus withstands the cross-flow generated as the coolant turns from the axial to the radial directions, and provides for shielding the flow distribution in the upper internals ff the vessel.
Advanced design pressurized water reactors of the type here considered incorporating such a calandria assembly are disclosed in the co-pending U.S. patent applications: Ser. No. 490,101 to James E. Kimbrell et al., for "NUCLEAR REACTOR"; U.S. patent application Ser. No. 490,059 to Luciano Veronesi for "CALANDRIA"; and U.S. patent application Ser. No. 490,099, "NUCLEAR REACTOR", all thereof concurrently filed on Apr. 29, 1983 and incorporated herein by reference.
Maintenance of suhh reactors, for example, requires that the upper closure dome be removed to provide access to the calandria assembly which in turn is removed to afford access to the WDRC and RCC rod clusters for repair or replacement, and as well to the core for rearrangement or replacement of the fuel rod assemblies. To accomplish this, the calandria assembly typically is removable from the inner barrel assembly, withdrawing thereby the WDRC and RCC rods from within the corresponding rod guides.
As before noted, the rod guides for each of the RCC and WDRC rod clusters are mounted rigidly at their bottom ends to the upper core plate, preferably by being bolted thereto, and extend in parallel axial relationshi to dispose the upper, free ends thereof adjacent the lower calandria plate. This cantilever-type mounting is necessitated to accommodate axial (i.e., vertical) movement of the free ends of the rod guides, which occurs due to thermal expansion and thus axial elongation of the rod guides, and fixed end motion caused by vibration and/or flexing of the upper core plate to which the bottom, fixed end of the rod guides are mounted.
Because of these factors, it is not possible to rigidly and permanently secure the free ends of the rod guides to the lower calandria plate. Preferably, the design of the pressure vessel and particularly of the support structures which mount the free ends of the rod guides to the lower calandria plate permit both the assembly and removal of the calandria, relatively thereto, without special tools. Nevertheless, the mounting means for the free ends of the rod guides not only must constrain the same against lateral motion due to vibration, flow and thermal forces while accommodating the aforesaid axial movement of the free ends of the rod guides, but also must avoid wear of the reactor internals arising out of loads imposed on the guides and the previously discussed axial motion of the free ends of the guides.
In some existing designs and as used with conventional reactors, split pins are employed at the free ends of the rod guides for restricting lateral motion while permitting a limited extent of axial motion. Such designs, however, present wear concerns for the reasons above-noted. In fact, due to the high loads and large axial motion of the free ends in the advanced design pressure vessels, the use of split pins for the free end supports is deemed not practical.
There thus exists a substantial need for a top end support structure for the top, free ends of the rod guides in such advanced design reactors, which satisfies these complex structural and operational requirements but yet which is of simple design and employs a minimum number of parts, thereby to achieve cost economies both in the construction and also in the maintenance of such reactors.